1. Field of the Invention
The present invention relates generally to amplifiers, and specifically to switching amplifiers.
2. Description of Related Art
Switching amplifiers typically generate a pulse-width modulated (PWM) waveform by comparing a triangular oscillator's waveform to the audio input signal. The linearity of the triangular waveform has a direct effect on the distortion of the audio output. Many other elements of a switching amplifier such as fluctuation of power supply voltages, switching delays, dead times, nonlinearities of most semiconductor components and real passive components, all contribute to relatively high distortion of switching amplifiers.
Reduction in distortion is normally accomplished with negative feedback. In a switching amplifier there is normally a reconstruction filter to minimize EMI at the amplifier output. The low-pass nature of the reconstruction filter introduces a phase shift that quickly approaches and exceeds 180 degrees. That large phase shift makes the design of the feedback loop difficult due to the possibility of oscillation when the total phase loop shift exceeds 360 degrees, taking into account the inherent 180 degrees phase shift of the negative feedback path and the error amplifier's own phase shift which reaches 90 degrees at only tens of hertz. Very often the feedback signal is picked up from a switching node of the power switch, before the reconstruction filter, therefore not including the reconstruction filter, to circumvent the 180-degree phase shift of the reconstruction filter. A reconstruction filter typically uses an inductor and a capacitor. These devices have their own non-linearities especially when an iron or ferrite core is used in the inductor. Magnetic hysteresis is naturally non-linear, and can be a major cause of distortion of switching amplifiers.
The need for stable feedback loop thus limits the bandwidth of typical switching amplifiers. Some switching amplifiers that process signals only in the digital domain don't even have any feedback and suffer from distortions. Limited bandwidth of conventional feedback for switching amplifiers thus limits their audio performance in terms of distortion. Even all linear amplifiers designed with global feedback by necessity have a dominant pole to assure loop stability. That dominant pole in turn limits the bandwidth of linear amplifiers, and more importantly reduces their ability to attenuate distortion at high frequencies where human ears can really tell slight differences. Indeed studies have shown that high-order harmonic distortions are not pleasing to our ears whereas low order harmonics can be tolerated because they already exist in the audio program and because they are in fact harmonious with the fundamental. Tube amplifiers are often liked for their high second and third order harmonic distortions while their higher order harmonic distortions are often much smaller due to their class-A type of operation.
Prior art of self-oscillating switching amplifiers such as U.S. Pat. No. 6,707,337 senses mainly switching nodes and integrates their voltages to generate ramps that eventually force comparators to change state. Again the oscillator loop does not include the reconstruction filter therefore their audio performance may be left to be desired. Furthermore it requires both the audio signal and its inverted signal to operate. The signal inverter itself is a source of phase shift and distortion.
Thus there is a need for switching amplifier circuits capable of linear transfer function, high loop gain and bandwidth.